1. Field of the Invention
The present invention relates to a technique for transmitting signals and, more specifically, to an apparatus for transmitting a positioning signal, a positioning system including the apparatus, and to a system for transmitting a positioning signal.
2. Description of the Background Art
A satellite navigation system such as GPS (Global Positioning System), GLONASS (Global Navigation Satellite System), GALILEO or the like transmits a SS (Spread Spectrum) signal for measuring a distance to a satellite transmitting the signal. It is possible for the user to determine a position, using distances to four or more satellites. The SS signal for the satellite navigation system includes a carrier wave modulated by a pseudo-noise code. As for the length of the code, it is about 300 m for a C/A (Coarse and Access) code, and about 30 m for a P (Precision or Protect) code. The carrier wave has a wavelength of about 19 cm for the radio wave in a so-called L1 band, and about 24 cm for the radio wave in an L2 band. The code data has a unique pattern, and a receiving apparatus of a user can measure an exact distance represented by the number of chips from the apparatus itself to the satellite. The carrier wave itself is transmitted repeatedly, and the receiving apparatus of the user cannot find the number of waves from the apparatus itself to the satellite in the case of a so-called point positioning.
In the foregoing, conventional art related to the present invention has been described based on general technical information known to the applicant. To the best of applicant's memory, the applicant does not have any information to be disclosed as prior art, before the filing of this application.
Positioning using a code is realized through exact matching between a signal referred to as a replica generated in a receiving apparatus and a received signal. The receiving apparatus of a user can measure the distance to the satellite with the accuracy of up to about one hundredth of the code length or wavelength. A receiving apparatus for point positioning used in a so-called stand-alone manner is capable of positioning solely on the code data. Therefore, in that case, positioning accuracy is limited to a few meters. In order to use a phase of the carrier wave for positioning, the user must have an access to a measurement by another receiving apparatus, apply a special processing to find indefinite number of wavelengths to a satellite, and continuously track the wavelength. If a signal from the satellite should be shut off, the user must do the above-described processes again from the beginning.
Therefore, today, a user of point positioning uses positioning, knowing that the measurement based on the code transmitted from the satellite has limited accuracy.
In the so-called RTK (Real time kinematic) mode, by using a reference point, it becomes possible for the user to execute positioning using the phase of the carrier wave. In order to use such a method, however, at least five satellites must be visible to the user, and in addition, distances to at least four satellites must be continuously measured without interruption.
A process for eliminating ambiguity is not very reliable even today, and effective technique has not been proposed. Therefore, a user of a satellite navigation system cannot know for sure whether the position calculated based on the phase of the carrier wave is correct or not.
The present invention was made to solve the above-described problems, and its object is to provide an apparatus that can transmit a signal enabling higher accuracy of positioning.
Another object of the present invention is to provide an apparatus for transmitting a positioning signal that can transmit a signal attaining lower uncertainty of multiple solutions in the so-called point positioning.
A further object of the present invention is to provide a system including an apparatus that can transmit a signal enabling higher accuracy of positioning.
A still further object of the present invention is to provide a system transmitting, together with a positioning signal, information for correcting any error.